1. Field of the Invention
This invention relates to implants, and more particularly, to a spinal facet compression screw comprising a plurality of variable pitch thread zones and a buttress head.
2. Description of the Related Art
The field of spinal implantation burgeons with devices and methods for the achievement of fixation between adjacent vertebrae. The most common devices currently used for the fixation are pedicle screw systems. In a typical pedicle screw system, screws are placed in to the pedicles of adjacent vertebrae and are stabilized together using various separate rod or plate means. An emerging means for achieving fixation of adjacent vertebra is the use of trans-facet fixation. Several devices listed below achieve fixation by placement of a screw or other means though the facet joint. This procedure has the advantage of being significantly less invasive than pedicle screw procedures, since it does not require a separate rod or plate means and only requires two bilateral screws to achieve fixation per level, rather than four in a pedicle screw system. For these reasons, trans-facet fixation has been growing in popularity.
A key goal of trans-facet fixation is the achievement of firm and direct contact of the opposing facet joint surfaces. Such contact is required for the desired bony fusion to take place. The current state of the art relies on the simple tightening of a lag screw to achieve external compression of the facet. This has limited effectiveness due to the limited ability of the relatively fragile facet joint to withstand external screw-tightening forces.
Variable pitch screws have been used in orthopedic surgery, particularly trauma repair, in the past. This is exemplified by the Herbert screw, invented in 1976. These screws, however, rely only on internal compression, and do not benefit from the external screw head buttressing as described in the current invention.
Some of the systems for bone fixation relating to facet fusion are shown or known from U.S. Patent Publications 20030208202 to Falahee; 20040087948 to Suddaby; 20040254575 to Obenchain et al.; 20050124993 to Chappuis; 20050149030 to Serhan; 20050267480 to Suddaby; 20060111779 to Petersen; 20060111780 to Petersen; 20060200149 to Hoy et al.; 20060212034 to Triplett et al.; 20060264953 to Falahee; 20070112428 to Lancial; 20070233092 to Falahee; 20070233093 to Falahee; 20080234758 to Fisher et al.; 20080255618 to Fisher et al.; 20080255619 to Schneiderman et al.; 20080255622 to Mickiewicz et al.; 20080255666 to Fisher et al.; 20080255667 to Horton; 20080262555 to Assell et al.; 20080275454 to Geibel; 20090036926 to Hestad; 20090036927 to Vestgaarden; 20090036986 to Lancial et al.; 20090054903 to Falahee et al.; 20090076551 to Petersen; 20090093851 to Osman; 20090099602 to Aflatoon; 20090105819 to Barry; 20090112264 to Lins; 20090125066 to Kraus et al.; 20090131986 to Lee et al.; 20090163920 to Hochschuler et al.; 20090177205 to McCormack; 20090187219 to Pachtman et al.; 20090192551 to Cianfrani et al.; 20090216273 to Cox; 20090234394 to Crook; 20090234397 to Petersen; 20090248082 to Crook et al.; 20090248089 to Jacofsky et al.; 20090264928 to Blain; 20090270929 to Suddaby; 20090275954 to Phan et al.; 20090275992 to Phan et al.; 20090275993 to Phan et al.; 20090275994 to Phan et al.; 20090299412 to Marino; 20090306671 to McCormack et al.; 20090312763 to McCormack et al.; 20090312798 to Varela; 20090312800 to Chin et al.; 20090318980 to Falahee; 20100076490 to Greenwald et al.; 20100082065 to Butler et al.; 20100087859 to Jackson; 20100094356 to Varela et al.; 20100100135 to Phan; 20100114175 to McKay;
Other systems are shown in U.S. Pat. No. 7,708,761 issued to Petersen; U.S. Pat. No. 7,699,878 issued to Pavlov et al; U.S. Pat. No. 7,608,094 issued to Falahee; U.S. Pat. No. 7,563,275 issued to Falahee et al.; U.S. Pat. No. 7,452,369 issued to Barry; U.S. Pat. No. 7,223,269 issued to Chappuis; U.S. Pat. No. 6,648,893 issued to Dudasik; U.S. Pat. No. 6,540,747 issued to Marino and U.S. Pat. No. 6,485,518 issued to Cornwall et al.
Therefore, what is needed is a new device which draws together the opposing facet joint surfaces via internal compression in addition to external compression.